1. Field of the Invention
The present invention relates to an offset estimation device, and in particular to an offset estimation device for use in a communication device capable of effectively correcting the symbol timing and frequency offset of received data.
2. Description of the Background Art
The PHS (Personal Handy Phone System) is a typical wireless communication system on the basis of TDMA (Time Division Multiple Access). Demodulators for use in the PHS system are provided with a frequency offset correction circuit. The frequency offset correction circuit makes it possible to accurately demodulate a received signal with its frequency offset removed.
The PHS system uses TDMA/TDD (Time Division Multiple Access/Time Division Duplex). The frame format of TDMA/TDD is of a 5 msec frame length composed of eight time slots each of 0.625 msec long. The first to fourth ones of the time slots are assigned to transmission use and the fifth to eighth time slots to reception use.
For use in communication between a base station, or cell station (CS), and a terminal device, or personal station, (PS) in the PHS system, there are two types of channels, i.e. control channel (CCH) and traffic channel (TCH).
The control channel is for use in performing necessary processes, such as handshaking, synchronization, PS location registration, PS authentication process, in a call connection procedure before actual data transmission and reception between cell and personal stations. On the other hand, the traffic channel is for use in performing actual data transmission and reception between cell and personal stations. The control and traffic channels are used for respective purposes different from each other in this manner. Because of this, the control and traffic channels are also different from each other in configuration of time slots to be transmitted over the respective channels.
As an example of TDMA communication, the communication of a PHS system will be described which is in conformity with ARIB STD-28 (Association of Radio Industries and Businesses Standard-28). A time slot transmitted over a control channel will be referred to as a physical slot for control, and a time slot transmitted over a traffic channel will be referred to as a physical slot for communication. The formats of the physical slot for control and the physical slot for communication in accordance with DQPSK (Differential Quadrature Phase Shift Keying) are defined as follows.
The format of the physical slot for control has its preamble defined longer so as to ensure frequency offset estimation for the purpose of transmitting and receiving control data in advance of actual communication to establish synchronization, for example. Specifically, the physical slot for control is composed of a ramp field of 4 bits, a preamble field of 64 bits, a unique word field of 32 bits, a payload field of 108 bits, a CRC (Cyclic Redundancy Check) field of 16 bits, and a ramp field of 4 bits. By contrast, the physical slot for communication is composed of a ramp field of 4 bits, a preamble field of 8 bits, a unique word field of 16 bits, a payload field of 180 bits, a CRC field of 16 bits, and a ramp field of 4 bits. The physical slot for communication is for use in carrying data to be actually transmitted and received rather than control data, and is therefore provided with a longer payload field. For this purpose also, the preamble field is defined to be shorter.
In the format of the DQPSK modulation scheme, the time slot configuration differs in preamble length between the physical slot for control and the physical slot for communication, thus giving rise to different demodulation schemes used. Since the physical slot for control has its preamble length long, sufficient data can be used for estimating the frequency offset in the demodulation process. Because of this, only with the preamble information of one slot, a highly reliable value can be obtained as an estimated frequency offset, and on the basis of this information the subsequent unique word and information can be correctly demodulated.
If the bit length necessary for accurately estimating a frequency offset is 40 bits, the physical slot for control has the long preamble field of 64 bits which is sufficient for estimating the frequency offset in the demodulation process. It is therefore possible to accurately demodulate the subsequent unique word, payload and CRC data. By this demodulation, it is possible to use information on the frequency offset estimated in respect of the first time slot transmitted in each of the frames.
Contrary to this, since the preamble in the slot of the traffic channel is very short as compared with that of the control channel, mere information on the preamble of one slot cannot render a reliable value as an estimated offset so that the demodulation is not accurate. The demodulation of a traffic channel is therefore performed in such a fashion that the frequency offset estimated in each slot of a traffic channel is applied to correcting the frequency offset in the slots of the traffic channel following thereto. A repetition of this process until correct data are obtained renders the demodulation accurate.
However, when abrupt degradation occurs due to fading or the like in the communication environment during the communication between a cell station and a personal station over a traffic channel, the frequency offset, when estimated on the basis of received signal thus degraded, becomes more likely to be erroneously estimated. In the case of traffic channel, for the correction of the frequency offset in the first slot of the next frame, the frequency offset thus erroneously estimated in the previous frame is used so as to render the estimation in the next slot less reliable. This estimation at the first slot of each frame is repeated, resulting in increasing the possibility of failing to obtain accurately demodulated data.